Add new GtkActionable interface

This is the interface for GtkWidgets that can be associated with an
action on a GtkAppicationWindow or associated GtkApplication.
It essentially features 'action-name' and 'action-target' properties
with some associated convenience API.
This interface is implemented by GtkButton and GtkToolButton.
https://bugzilla.gnome.org/show_bug.cgi?id=667394